Magnetic recording media are widely employed in recording tapes, video tapes, floppy disks, and the like. In magnetic recording media, a magnetic layer in which a ferromagnetic powder is dispersed in a binder is laminated on a support.
The magnetic recording media must have a variety of high-level characteristics, such as electromagnetic characteristics, running durability, and running performance. That is, in audio tapes used to record and reproduce music, a greater ability to reproduce sound sources is required. Further, good electromagnetic characteristics such as the ability to reproduce a source image are required of video tapes. In addition to possessing such good electromagnetic characteristics, magnetic recording media must also have good running durability. The improvement of binder dispersibility has been carried out as one approach for achieving good running durability.
For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-39639 discloses polyester polyurethane that contains polyester polyol comprising an aliphatic dibasic acid and a specific branched aliphatic diol, and describes the objectives of improving the dispersion stability of powder, improving output, preventing head fouling, suppressing increased dropout (DO), and improving long-term preservation properties (improvement in the μ value (coefficient of friction) after one week at 60° C. dry). Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-39639 is characterized by the use of high-strength, highly dispersible polyurethane as binder. However, it presents problems in that it requires the additional use of vinyl chloride resin incorporating polar groups to adjust the viscosity of the coating liquid and the physical properties of the tape, head corrosion is caused by the generation of hydrochloric acid, and the environment is compromised when the magnetic recording medium is discarded.
As environmentally safe binders, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-307734 discloses a binder comprising just two or more polyester urethanes having different Tg values, and Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-52539 discloses a binder comprising just polyester urethane with the low molecular weight component removed to achieve a Tg of 60 to 80° C. Although Japanese Unexamined Patent Publications (KOKAI) Heisei No. 5-307734 and Heisei No. 6-52539 provide magnetic recording media that are environmentally safe, the disclosed polyurethane resins afford inadequate dispersibility, smoothness, and electromagnetic characteristics. Preservation properties are also inadequate.
When magnetic recording media are employed as the data media of recent years that have been required to deliver high capacity, high speed, and high reliability, the trace amounts of hydrochloric acid gas that are generated by the magnetic recording media are known to have the potential not only to adversely affect the preservation properties of the tape but also to cause head corrosion. Deterioration in characteristics due to corrosion is of particular concern in the magnetoresistive (MR) heads employed in computer data recording systems, and the handling of such corrosion has become an issue.
When a tape is stored for an extended period at high temperature under high humidity, the degradation of materials in a tape by hydrochloric acid gas, such as the generation of fatty acids through the hydrolysis of ester lubricants causes. The resulting fatty acid sometimes transfers to the surface of the magnetic layer to precipitate and causes crystallization. Since the recording density becomes higher than that of prior art, the effects of even minute quantities of foreign matter present a significant problem.
Taking this problem into consideration, Japanese Unexamined Patent Publication (KOKAI) No. 2001-176052 proposes a magnetic recording medium employing a binder containing polyurethane of prescribed structure and specifies the chlorine content of the magnetic recording medium. However, although the magnetic recording medium described in Japanese Unexamined Patent Publication (KOKAI) No. 2001-176052 affords good coating smoothness and long-term preservation property, it does not provide adequate reproduction output and requires further improvement in electromagnetic characteristics, particularly in recording and reproduction systems in which MR heads are employed.
Therefore, it is an object of the present invention to provide a magnetic recording medium that exhibits excellent electromagnetic characteristics especially in recording and reproducing systems using an MR head, as well as has excellent long-term preservation property.